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As a living, changing thing, our body can gain or lose weight depending on circumstances. This process of weight gain or weight loss is determined by the energy-intake / energy expenditure equation - otherwise known as calories-in, calories-out.
Energy in this case is measured in units called calories. We take in energy by eating or drinking anything that contains calories. If we eat a bagel containing 200 calories, our energy intake is 200 calories.
Energy expenditure is made up of three things. 1) Our Basal Metabolic Rate (BMR). On average this accounts for about 60 percent of all energy expenditure, and is the amount of energy or calories our body consumes on basic body processes alone, without any additional physical activities. BMR varies according to several factors, especially the fat-muscle ratio of our body and the amount of exercise we take, but on average, our basal metabolic rate burns about 11 calories of energy per pound of body weight (women), or 12.2 calories per pound of body weight (men). 2) Physical Activity. On average this accounts for about 30 percent of energy expenditure. 3) Dietary Thermogenesis. This is the energy or calories we burn as we eat and digest food. It accounts for about 10 percent of the total energy we expend.
Like a see-saw, if our energy (calorie) intake balances our energy (calorie) expenditure, our weight remains stable. If we take in more energy (calories) than we use up, we gain weight. If we take in less energy (calories) than we burn, we lose weight. This is the basic law of weight gain and weight loss.
If, as above, we take in more energy (calories) than we burn, this surplus energy (calories) is stored as body fat. Although different foods are metabolised and processed in the body in different ways, fat cells are the ultimate destination for all nutrients your body does not need for immediate use.
If, as above, we take in less energy (calories) than we burn, our body re-converts some of it's stored body fat back into energy. Over time, this leads to weight loss.
Obesity expert
Jules Hirsch wrote in the British Medical Journal: "The balance of
food intake and energy expenditure that maintains constant energy storage
is determined by the metabolism of muscle, liver, pancreas and intestine.
The balance is regulated by the adrenal and sex steroids as well as adipose
tissue itself, which together create a complex set of signals [which affect]
energy dissipation and food intake. Hovering over this complex system
are potent psychosocial and behavioral factors."
In other words, even though the relationship between energy intake and
energy expenditure is clearly marked, it is affected by a huge number
of bodily and mental processes.
Although there are a number of complex factors that influence the energy-intake vs. energy-expenditure equation, no one seriously challenges the basic equation between calories-in and calories-out. So if you want to lose fat and keep your weight under control, aim to reduce your calorie-intake below your calorie expenditure. This is best achieved by a combination of sensible diet and regular exercise.
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